1. Field of the Invention
The present invention relates to a liquid crystal display device in which alignment of liquid crystal can be controlled by applying an electric field along the substrate face, and relates to a structure in which in addition to a wider angle of view, a higher aperture ratio can be achieved.
2. Description of the Prior Art
Recent TN mode liquid crystal display devices have a problem of high dependency on the angle of view, since the visibility in the vertical direction is inferior in spite of excellent visibility in the lateral direction. The applicant of this application claimed liquid crystal display devices having a structure by which the above problem can be solved in Japanese Patent Application Nos. 7-1579, 7-306276, and the like.
According to the techniques described in such Patent Applications, instead of providing liquid crystal driving electrodes for each of the upper and lower substrates sandwiching the liquid crystal layer, two types of linear electrodes 12 and 13 having different polarity from each other are provided only on the lower substrate 11 at a distance from each other, as is shown in FIG. 10, and no electrode is formed on the upper substrate 10 shown in the upper side of FIG. 10 so that liquid crystal molecules 36 are aligned in the direction of the transverse electric field (in the substrate-face direction) which is generated between the linear electrodes 12 and 13 by applying a voltage.
In more detail, as is shown in FIG. 9, the linear electrodes 12 are connected by a base line 14 to form a comb-shaped electrode 16, the linear electrodes 13 are connected by a base line 15 to form a comb-shaped electrode 17, the comb-shaped electrodes 16 and 17 are engaged with each other such that the linear electrodes 12 and 13 are alternately positioned without being in contact with each other, and a switching element 19 and a power source 18 are connected to the base lines 14 and 15.
As is shown in FIG. 11A, an alignment film is formed on the liquid-crystal side of the upper substrate 10 to align the liquid crystal molecules 36 in the .beta. direction, another alignment film is formed on the liquid-crystal side of the lower substrate 11 to align the liquid crystal molecules 36 in the .gamma. direction parallel to the .beta. direction, and a polarizing plate polarizing light in the .beta. direction shown in FIG. 11A and a polarizing plate polarizing light in the .alpha. direction are provided for the substrates 10 and 11, respectively.
According to the above structure, the liquid crystal molecules 36 are homogeneously aligned in the same direction when no voltage is applied between the linear electrodes 12 and 13, as is shown in FIGS. 11A and 11B. In this state, a light beam transmitted through the lower substrate 11 is polarized in the .alpha. direction by the polarizing plate, passes through a layer of the liquid crystal molecules 36, and then reaches the polarizing plate of the upper substrate 10, which polarizing plate has a polarization direction .beta. different from the direction .alpha.. The light beam is thereby shaded by the polarizing plate of the upper substrate 10 and is unable to pass through the liquid crystal display device, thereby rendering the liquid crystal display device in a dark state.
When a voltage is applied between the linear electrodes 12 and 13, among the liquid crystal molecules 36, those adjacent to the lower substrate 11 are aligned perpendicular to the longitudinal direction of the linear electrodes 12 and 13. The nearer a liquid crystal molecule is located to the lower substrate 11, the more strongly this phenomenon is observed. In other words, lines of electric force perpendicular to the longitudinal direction of the linear electrodes 12 and 13 are generated by the transverse electric field (an electric field in the substrate-face direction) produced by the linear electrodes 12 and 13. Thus, the major axes of the liquid crystal molecules 36 aligned in the .gamma. direction by the alignment film formed on the lower substrate 11 are altered to the .alpha. direction, i. e., perpendicular to the .gamma. direction, by the force of the electric field which is stronger than that of the alignment film, as is shown in FIG. 12A.
Therefore, twisted alignment is achieved in the liquid crystal molecules 36 by applying a voltage between the linear electrodes 12 and 13, as is shown in FIGS. 12A and 12B. In this state, the polarization direction of the polarized light beams, which have been transmitted through the lower substrate 11 and polarized in the .alpha. direction, is converted by the twisted liquid crystal molecules 36 so that the polarized light beams are allowed to pass through the upper substrate 10 having a polarizing plate whose polarization direction .beta. is different from the .alpha. direction. The liquid crystal display device thereby exhibits a bright state.
FIGS. 13 and 14 are an enlarged fragmentary view of the structure of an actual active-matrix liquid crystal driving circuit to which a liquid crystal display device equipped with the linear electrodes 12 and 13 is applied.
The structure shown in FIGS. 13 and 14 corresponds to only one pixel. On a transparent substrate 20 such as a glass substrate, a gate electrode 21 and linear common electrodes 22 both made of a conductive layer are separately provided parallel to each other. A gate insulating film 24 is formed to cover these electrodes. A thin-film transistor T is formed such that a source electrode 27 and a drain electrode 28 are formed on a portion of the gate insulating film 24 corresponding to the gate electrode 21, and a semiconductor film 26 is provided on a portion of the gate insulating film 24 between the source electrode 27 and the drain electrode 28. A linear pixel electrode 29 made of a conductive layer is formed on a portion of the gate insulating film 24 between the common electrodes 22.
FIG. 13 is a plan view of these electrode. Gate lines 30 and signal lines 31 are formed on the transparent substrate 20 according to a matrix pattern. The gate electrode 21 which is a part of the gate line 30 is provided at a corner of each pixel region formed by the gate lines 30 and the signal lines 31. Via a capacitor electrode 33, the drain electrode 28 above the gate electrode 21 is connected to the pixel electrode 29 which is provided between the common electrodes 22 in parallel with the signal line 31 and the common electrodes 22.
The ends, near the gate line 30, of the common electrodes 22 are connected by a connecting line 34, provided in the pixel region in parallel with the gate line 30, and the other ends of the common electrodes 22 are connected by a common line 35, provided in the pixel region in parallel with the gate line 30. The common line 35 is provided over numerous pixel regions in parallel with the gate line 30 so as to apply a common voltage to the common electrodes 22 provided for each pixel region.
As is shown in FIG. 14, on the surface, opposing the substrate 20, of the substrate 37, a light shielding matrix 38 is formed with an opening 38a corresponding to a pixel region, and a color filter 39 is also provided to cover the opening 38a.
In the above structure shown in FIGS. 13 and 14, lines of electric force generated by a transverse electric field can be obtained along the directions of the arrows a shown in FIG. 14. Thus, the liquid crystal molecules 36 are aligned by the transverse electric field in a manner shown in FIG. 14. The dark and bright states are thereby switchable by controlling the alignment of the liquid crystal molecules 36 similarly to the above description made with reference to FIGS. 11 and 12.
However, according to liquid crystal display devices having the above structure, the aperture ratio is disadvantageously reduced in spite of a wide angle of view. In other words, although the liquid crystal molecules 36 are aligned by the transverse electric field generated between the pixel electrode 29 and the common electrodes 22 in the structure shown in FIGS. 13 and 14, in regions above the common electrodes 22, the direction of the electric field applied to the liquid crystal molecules 36 differs from that of the transverse electric field, and thus the alignment direction of the liquid crystal molecules 36 in the regions above the common electrodes 22 is different from that in the region between the pixel electrode 29 and the common electrodes 22, as is shown in FIG. 10.
Therefore, as is shown in FIG. 14, the light shielding matrix 38 is conventionally employed for shading the regions above the common electrodes 22, which regions may cause problems such as light leakage. Furthermore, the periphery of the opening 38a of the light shielding matrix 38 is positioned slightly inside the inner end 22a of each common electrode 22, thereby increasing the region shaded by the light shielding matrix 38. Thus, the aperture ratio of the resulting liquid crystal display device cannot be increased.